Okay, I have read the FAQ and all that stuff on how this is supposed to be done. Here's the problem. No matter what I do, the idle speed is not correct. I doubt my tach is off so far as to cause me to set the idle speed about 500 RPM too low when cold. If I set the idle RPM at operating temp (about 190* F, just inside closed loop range), the car will NOT idle when cold, and dies as a result unless I step on the gas. If I set the idle RPM when cold, then the RPM is too high when hot. I am trying like mad to reach the factory setting of 700 RPM at operating temperature with the car in gear (auto trans, 1987 model year). I have 16+" of manifold vacuum, no air leaks, a good MAF, CTS, IAT, ign module, and just about everything else that can affect the idle speed, and my plugs are normal grayish-tan color.
I have noticed that with one of my ECM's (both work) the IAC does not have control over the idle speed. The other (with my original PROM set for the cold start injection system, which has been deleted) has a good IAC driver.

Okay, now I guess I should ask this question: Where does the IAC get its air source from for the 1987 model year? It seems that on my engine (using the 2.8 gear, minus the fuel rail, on the top end), the IAC gets most of it's incoming air from the PCV inlet air hose thing, or whatever GM calls that black piece of nylon pipe running from the rear of the intake manifold to the TB. I have removed that pipe, in search of one from a FIREBIRD (if anyone can help me out here it would be appreciated) because it originally broke on me and the seal I had on the PCV hose finally broke through for the last time, and I have a plug in the TB nipple, which means that air source is cut off.

 

well ill be the first to say that yeah your tach could be that far off, my camaro reads way high and my firebird reads way low, it idles at like 450 rpms lol, i dont think ive ever seen a factroy thirdgen tach read the right rpms

if it were me id set it to where it runs the best and not off the factory tach

 

The thing is, it also hunts for idle speed. That is, of course, unless I set the hot idle RPM for about 1500 no load... Then I can kiss my little mileage goodbye...

 

Actually you have it backwards. The IAC gets air from the throttle body (in front of the butterfly) and routes it through the pipe to the back of the intake manifold.
But, yes it does use that plastic pipe you took off!

 

Now, that does explain why the engine was a heck of a lot easier to start with the PCV pipe thing in place. Guess I'll have to clean the TB, AGAIN... At least I'll have an excuse to replace the TB mounting gasket that's been leaking .